a) Do you want to compare passages that are in Luke and Matthew but not in Mark (and vice versa)?

b) Do you want to use the web app to accomplish this?

In the desktop software I would use the Parallel Gospel Reader tool, but you could also just open the Eusebian Canons (provided that you have this resource in your library). The Eusebian Canons are an ancient gospel harmony. Canon V contains all passages that occur in Matthew and Luke (but not in the other two gospels) and canon VIII will give you all passages that occur in Luke and Mark (but not in Matthew and John).